1. Field of Invention
The present invention relates to a method for predicting the amount of sulfur oxides (Sox) stored at a denitrification (DeNOx) catalyst and an exhaust system using the same. More particularly, the present invention relates to a method for precisely predicting the SOx amount actually stored at the DeNOx catalyst and to an exhaust system which controls the regeneration timing of the DeNOx catalyst and the amount of reducing agent which is to be injected by using the method.
2. Description of Related Art
Generally, exhaust gas flowing out through an exhaust manifold from an engine is driven into a catalytic converter mounted at an exhaust pipe and is purified therein. After that, the noise of the exhaust gas is decreased while passing through a muffler and then the exhaust gas is emitted into the air through a tail pipe. The catalytic converter purifies pollutants contained in the exhaust gas. In addition, a particulate filter for trapping particulate matters (PM) contained in the exhaust gas is mounted in the exhaust pipe.
A denitrification catalyst (DeNOx catalyst) is one type of such a catalytic converter and purifies nitrogen oxides (NOx) contained in the exhaust gas. If reducing agents such as urea, ammonia, carbon monoxide, and hydrocarbon (HC) are supplied to the exhaust gas, the NOx contained in the exhaust gas is reduced in the DeNOx catalyst through oxidation-reduction reaction with the reducing agents.
Recently, a lean NOx trap (LNT) catalyst is used as such a DeNOx catalyst. The LNT catalyst absorbs the NOx contained in the exhaust gas when the engine operates in a lean atmosphere, and releases the absorbed NOx when the engine operates in a rich atmosphere. The release of the absorbed NOx from the LNT catalyst is called regeneration.
Since materials absorbing the NOx at the DeNOx catalyst, however, are alkaline materials, SOx (materials which are made by oxidizing sulfur components contained in a fuel or an engine oil) as well as NOx contained in the exhaust gas is also absorbed. Sulfur poisoning of the DeNOx catalyst deteriorates purification efficiency of the DeNOx catalyst. Accordingly, desulfurization processes for the DeNOx catalyst is necessary.
If SOx amount poisoned at the DeNOx catalyst is greater than or equal to a predetermined amount, the engine is controlled to enter desulfurization mode so as to release the SOx poisoned in the DeNOx catalyst according to a conventional desulfurization method for an exhaust system. At this time, since the driving state of the engine, the temperature of the exhaust gas, the lambdas of inlet and outlet of the DeNOx catalyst should be considered, a lot of engine control unit (ECU) memories may be necessary and processing speed may be slow
In addition, temperature difference between the inlet and outlet of the DeNOx catalyst is large when desulfurization. It is difficult to select a reference temperature for desulfurization and a great many variables and maps are required.
Therefore, desulfurization is controlled by using the inlet temperature and lambdas of the DeNOx catalyst while neglecting changes in the driving state of the engine. Accordingly, aging of the DeNOx catalyst and fuel economy may be deteriorated.
The information disclosed in this Background section is only for enhancement of understanding of the general background of the invention and should not be taken as an acknowledgement or any form of suggestion that this information forms the prior art already known to a person skilled in the art.